The Turkey-backed rebels on Thursday drove Islamic State from al-Bab, the jihadist group's last significant stronghold in northwest Syria, along with two smaller neighboring towns of Qabasin and al-Bezah, after weeks of street fighting.

The car bomb struck military and security offices in Sousian, which is behind the rebel lines about eight km (five miles) northwest of al-Bab, the Syrian Observatory for Human Rights, a Britain-based war monitor, said.

On Thursday, several Turkey-backed rebels were killed by a mine in al-Bab while clearing the town of unexploded ordnance after Islamic State retreated, the Observatory said.

Syria's main conflict pits President Bashar al-Assad, backed by Russia, Iran and Shi'ite militias, against rebels that include groups supported by Turkey, the United States and Gulf monarchies.

However, both those sides, as well as a group of militias led by Kurdish forces and supported by the U.S., are also fighting Islamic State, which holds large swathes of northern and eastern Syria.

Turkey directly intervened in Syria in August in support of a group of rebel factions fighting under the Free Syrian Army banner to drive Islamic State from its border. It also wants to stop Kurdish groups from gaining control of most of the frontier.

After taking al-Bab on Thursday, Turkish forces shelled Islamic State in the smaller neighboring town of Tadef, the Observatory reported. The area immediately to the south of Tadef is held by the Syrian army and its allies.